The present invention relates to a tool for stitch forming machines, particularly knitting machines, the tool including a shank adapted to be inserted into a guide groove (trick) in a bed and provided with stitch forming means at least in the region of one end. The shank is bent transversely to its longitudinal direction and includes at least one butt.
Such tools are needles, particularly latch needles, compound needles, hooks, sinkers and similar components which are punched out of steel sheet (steel band) or are manufactured from steel wire and contribute to the forming of stitches. They are guided so as to be longitudinally displaceable in the guide grooves of the associated bed, with their longitudinal movement being controlled by cam elements which cooperate in a known manner with the needle butts.
Due to the continual development of new stitch forming machines having increased operating speeds, tools for these machines, for example the latch needles of small circular knitting machines, are not only subjected to greater mechanical stresses than encountered in earlier such machines but the speed with which the tools are moved back and forth is also increased to the same extent as the operating speed. For example, in knitting machines, an increased needle velocity results in greater shimmying movements of needles in guide grooves of the needle bed which in turn cause needle vibrations. This interferes with the proper and uniform sliding movement of the needles in the guide grooves of the needle bed and has an adverse effect on the quality of the merchandise produced.
At the same time, the vibrations caused by the shimmying movements generate transversely oriented bending vibrations at a stitch forming portion of the needle which projects from the needle bed such that the head of the needle, together with the hook and the latch, are deflected from a plane of symmetry of the needle to such an extent that the latch brushes against an adjacent transfer jack during a pivoting movement of the needle. When this occurs, irregularities appear in the merchandise produced. Further, this brushing of the needle against the adjacent transfer jack produces a unilaterally ground noucat or latch spoon having sharp edges which may damage the yarn during knitting, thereby making the needle unusable immediately.
A known measure for counteracting such shimmying of the needle in its guide groove in the needle bed is provision of a so-called friction bend in the needle shank. Such a friction bend is formed by laterally bending the shank at one location in a direction which is transverse to the longitudinal extent of the shank, such that the shank regions on either side of the friction bend are disposed at an obtuse angle relative to each another (as specified in DIN ISO 8119, Part 1, Page 28, No. 14). Due to this friction bend, when the shank is inserted into a guide groove of the needle bed, it is pressed laterally against a side wall of the guide groove with a certain elastic bias.
As disclosed, for example, in the introduction to the specification of German Auslegeschrift No. 2,225,834, which corresponds to U.S. Pat. No. 3,977,217, the basic problem of the above-mentioned prior art friction bend is that the bias created in this way is not sufficient to keep the needle shank lying against the side wall, as seen in the longitudinal direction of the needle, over the entire length of the portion of the needle disposed in the guide groove of the needle bed. This is illustrated in FIG. 4, showing a prior art needle lying in a guide groove 11 having a guide groove side wall 14. The prior art needle includes a shank 1' and a needle head 3', the shank 1' having a friction bend 17. The needle head 3' is separated from the guide groove side wall by a distance or separation S. During operation, the needle executes a longitudinal movement, during which the needle head 3' and the portion of the needle shank 1' following it are pushed out of the guide groove 11 of the needle bed. Due to the separation S in the position of the needle shown in FIG. 4, however, the longitudinal movement of the needle from the guide groove 11 causes the needle head 3' to perform an arcuate movement in a direction which is transverse to the longitudinal direction of the guide groove 11, i.e., in the down direction in FIG. 4. During this longitudinal movement of the needle, the bias with which the needle shank 1' is pressed against the associated the guide groove side wall 14 continuously changes. On the other hand, during this longitudinal movement of the needle, the needle head 3' moves toward the adjacent transfer jack so that, in the case of a fine gauge needle having a needle latch, the needle latch may engage with or rub against the adjacent transfer jack in an undesirable or improper manner.
In the German Auslegeschrift No. 2,225,834, in order to attempt to overcome the aforementioned problems, instead of providing the prior art friction bend, the shank itself is twisted about its longitudinal axis. According to this arrangement, only a section of the shank (not shown in FIG. 4) is twisted, and this twisted section is disposed in a region of the shank which remains in the guide groove 14 during the movement of the stitch forming tool. Due to the presence of this twisted section, the shank portion exiting from the guide groove 11 is held approximately in the middle of the guide groove, i.e. at a distance from the side walls of the guide groove, during the back and forth movement of the tool. However, the section remaining in the interior of the guide groove 11 is not held in the middle of the guide groove 11. Due to the lack of a lateral support of the shank portions lying in the front region of the guide groove, needle vibrations cannot be avoided during operation. Aside from this, in the twisted section there is only a linear contact between the side walls of the guide groove 11 and the twisted shaft portions which are urged under an elastic bias against the side walls of the guide groove 11. This linear contact extends essentially in the longitudinal direction of the guide groove 11 between the shank and the side walls of the corresponding guide groove 11. However, due to the elastic bias between the shank and the side walls of the guide groove 11 and the relatively small contact area therebetween, such linear contact causes relatively high pressures between the shank and the side walls of the guide groove 11, and consequentlY causes relatively greater wear. After relatively short periods of operation, the contacting edges are worn off so that lateral play develops between the side walls of the corresponding guide groove 11 and the shank, thereby causing vibrations and shimmying of the needle.
Finally, latch needles for knitting machines are disclosed in German Patent No. 2,238,196, in which a needle is shown having a needle shank which is corrugated several times starting at the butt of the needle and extending over its entire length so that its flank contacts the guide channel or groove cheeks in a manner which changes several times. The thus created serpentine course of the needle shank is intended to prevent a linear propagation of control shock pulses emanating from the butt and directed toward the needle head and thus to prevent or reduce the occurrence of premature hook breaks. Although in this latch needle the alternating flank contact in principle results in a bilateral guidance of the needle shank along both guide channel side walls or guide groove cheeks, close tolerances are required for the corrugations and the guide channels or guide grooves of the needle bed which cannot always be provided in practice.